Cytospora canker is caused by _Cytospora chrysosperma_. This fungus
attacks more than 70 species of hardwood trees and shrubs as well as
some conifers. Poplars and willows are among the most common hosts, and
are attacked throughout their range. Trees used for shade or windbreak,
and also cuttings in propagation beds are particularly susceptible to
this canker. This disease is most severe on trees growing under stress,
such as those growing on an unfavorable site, or injured by drought,
frost, fire, or severe pruning. The fungus is a normal inhabitant of the
bark and becomes parasitic only when the tree is weakened.
[Illustration: Cytospora canker on cottonwood.]
A canker begins as a gradual, circular killing of the bark of a limb or
stem. This infected tissue soon appears as a brownish, sunken patch,
around which the host often produces raised callus tissue. Small black
fungal structures (pycnidia) appear as small pimples on the dead bark.
During moist weather, thin threads of spores exude out from the
pycnidia. The inner diseased bark becomes dark and odorous. The cambium
is killed and the sapwood becomes watery and reddish brown as it becomes
infected. Water spouts may form below the canker before the tree dies.
Since this disease is most severe on weakened trees, shade trees should
be watered and fertilized to maintain healthy, vigorous growth. Wounding
and severe pruning should be avoided. Cuttings should be selected from
healthy planting material grown in disease free areas. If cuttings are
to be stored for any length of time, they should be kept at a
temperature below 35°. This low temperature will keep new infections
from occurring, even if spores are present.